I'm not exactly sure what purpose the vents serve, but I do know that the gas tube is really just a piston shroud/guide. Gas pressure is applied to the piston inside the gas block.

There's a video from the Ultimak guys floating around here somewhere. They purposely mounted a too-short Ultimak gas tube on an AK. There was something like a 1/4-3/8" gap between the front of the gas tube and rear of the gas block. Guess what? The AK functioned just fine.

Well, I know the (or one of) the holes' purpose is to release gasses out front, instead of out into the receiver. It probably makes more of a difference in Russian/Chinese/Romanian battlefield, when you get to clean your rifle every second time you get to sleep in an actual bed under a dry roof, and you're building up crud.

And I knew that the majority of the pressure builds in the gas block, not the tube. But I don't know if that pressure keeps acting on the piston through the full length of its travel in the tube, or if it goes around in the ribs quick enough not to, and if vents would make a difference in that.

If they don't, I'll probably leave it alone. Less area for dirt to get in.

Ian

October 7, 2008, 12:16 PM

IIRC, the vented gas tube was the early style. Later in the production history they changed the design a bit for cost and ease of manufacture. The later ones are still vented, but the holes are semi-circular and cut into the circumference of the gas block instead of the length of the gas tube. Take a look at any Romanian AK, for instance, and you'll see them right where the gas tube meets the gas block.

Deus Machina

October 7, 2008, 12:38 PM

I've seen those, and it makes sense. If it's for gas to escape, it's partially covered by the gas tube.

My Saiga doesn't have those. Had the gas tube off when I was fiddling with the foregrip, and the block's smooth all the way around.

nalioth

October 7, 2008, 01:58 PM

But I don't know if that pressure keeps acting on the piston through the full length of its travel in the tube, or if it goes around in the ribs quick enough not to, and if vents would make a difference in that.
The gas tube is only there to guide the piston (reference the aforementioned Ultimak video). All the action happens in the gas block, no further impetus is provided via the gas tube.

On most AKs I've seen, the gas tube doesn't fit very tightly to the gas block anyway, regardless of the presence of vent holes.

Picard

October 7, 2008, 02:12 PM

The gas tube does not make a seal with the piston, so the holes are only for excess pressure to quickly leave the gas tube. Like mentioned earlier, the piston makes a seal with the gas block and loses that seal as soon as it begins to move back.

The gas tube is only there to guide and protect the gas piston, and to keep crud from entering into the receiver.